All I've really used for activating monsters is activated by nearby enemies. It's worked better than having the player shoot enemies to wake them up. In the last map I made, Polygons, Platforms, and Duct Tape, I tried using a visible monster trigger to activate some enemies on the ledge by having them teleport in. Stepping on the trigger seemed to not only activate those units, but also some others behind some doors. Not sure why it did that.

I was wondering if anybody could specificly tell me how the visible, invisible, and dual monster triggers work. Knowing this infomation would help me make better stuff.Thanks in advance.

Also: Invisible works with teleporting monsters only. Dual works with both visible and potential (not teleported-in yet). If you want doors to be unopenable by aliens, de-check that box in the door options.